Recently, the use of alternative fuel other than conventional petroleum-based fuel, in particular, biomass fuel is attracting attention in order to address diversification of energy sources and prevention of global warming. Of the biomass fuel, those made from animal or vegetable fat or waste edible oil, such as rapeseed oil, soybean oil, palm oil, coconut oil, sunflower oil, Jatropha oil, beef tallow, lard, and fish oil, have such characteristics as containing less sulphur and being a carbon neutral fuel, so that these fuels are attracting attention as alternative fuel for petroleum-based diesel.
Triglycerides, which are main ingredient of fat such as rapeseed oil, soybean oil, palm oil, coconut oil, sunflower oil, Jatropha oil, have a high viscosity. When the untreated triglycerides are used to a diesel engine, they tend to form a deposit in an engine or an injection valve. In contrast, fatty acid alkyl esters, which are obtained by transesterificating the triglyceride with a lower alcohol such as methanol or ethanol in the presence of homogeneous catalyst such as potassium hydrate, are called as biodiesel oil. Biodiesel oil has a low viscosity, and is used as alternative fuel for petroleum in diesel engine. In addition, there is an application to add and mix a certain amount of biodiesel oil with petroleum-based diesel engine fuel.
Standards for the properties of 100% biodiesel fuel used as the alternative fuel for petroleum are JIS K2391 (Japan), EN14214 (Europe), and ASTM D6751-09 (USA). In contrast, for mixed oil of biodiesel fuel and petroleum diesel (hereinafter, sometimes referred as biodiesel mixed or blended fuel), Act on the Quality Control of Gasoline and Other Fuels was revised (hereinafter, referred as revised Act on the Quality Control) and fully implemented in 25 Feb. 2009 in order to ensure the quality.
The standard is strict in that the increment of acids by oxidation of biodiesel mixed fuel (the mixed ratio of biodiesel is at most 5 percent by weight) is below 0.12 mgKOH/g under more strict oxidation condition than that as defined in ISO Standard (ISO12205) for oxidative stability of middle distillate, i.e. under a forced oxidation condition of blowing pure oxygen gas at 115° C. for 16 hours, the increment of acids is evaluated the amount of potassium hydrate required to neutralize the acid with basic potassium hydrate.
In order to improve oxidative stability of biodiesel fuel, technologies of adding some antioxidant to the fuel are known. Several technologies, such as adding propyl gallate as an antioxidant (Patent literature 1), adding a quinoline antioxidant or a phenylenediamine antioxidant (Patent literature 2), and adding a hindered amine antioxidant and a hindered phenol antioxidant (Patent literature 3) are reported.
Biodiesel fuel is easily oxidizable because it contains an easily autooxidizable unsaturated fatty acid. In particular, a polyunsaturated fatty acid having two or more carbon-carbon double bonds is extremely oxidizable, which causes formation of acids or sludge. Therefore, in the case of biodiesel fuel containing a large amount of unsaturated fatty acid, adding antioxidants to biodiesel fuel is not sufficient to improve oxidative stability so as to meet the standard within an economical range of additive amount.
Additionally, even if antioxidants are added to biodiesel fuel, the problem forming sludge due to polymerization of unsaturated bonds still can't be solved. There is a need of further technologies to improve oxidative stability of biodiesel fuel.
One method of decreasing the polyunsaturated fatty acid in biodiesel fuel with a poor oxidative stability is a method to hydrogenate its unsaturated bond with a hydrogenation catalyst. Generally, hydrogenation of fuel oil is often carried out under high hydrogen pressure. For example, hydrogenation catalysts for hydrocarbon oil and the method for producing and activating thereof are proposed in Patent literatures 4 and 5. However, extremely high pressure, for example 3.9 MPa, is required to hydrogenate in these technologies. In addition, Patent literature 6 describes a method for partial hydrogenating fatty acid alkyl ester oils. However, in the method, hydrogenation is carried out under extremely high pressure of 10 MPa, and used a combination of surfactant and homogeneous catalyst, which causes a problem of separating them from the product.
However, hydrogenation under high pressure is expensive to introduce and maintain the facility. Therefore, there is a need to develop a hydrogenation method without the need of pressure resistant facilities. Because local governments or small manufacturers often produce biodiesel oil, and they don't possess such a high pressure facilities, the hydrogenation under low pressure is particularly required. Additionally, it is also advantageous that realization of hydrogenation under low pressure allows resolving the difficulty in dealing with High Pressure Gas Safety Act.
Also, in terms of variation of the fatty acid component in biodiesel fuel due to hydrogenation, when the reaction of the fuel is carried out under high hydrogen pressure, the reaction can't stop at the mono-unsaturated fatty acid alkyl ester, and the ester is further hydrogenated to its saturated fatty acid alkyl ester. Saturated fatty acid alkyl esters have higher melting point than unsaturated fatty acid alkyl esters, so they have a poor fluidity at low temperature, they can't be used as fuel.
In order to address these problems, the inventors propose a hydrogenation catalyst capable of obtaining biodiesel fuel having an excellent oxidative stability by hydrogenating selectively a polyunsaturated fatty acid alkyl ester having two or more double bonds with a poor oxidative stability in the fatty acid alkyl ester contained in biodiesel fuel to obtain the mono-unsaturated fatty acid alkyl ester with relatively good fluidity at low temperature and oxidative stability (Patent literature 7).